1. Field of the Invention
The present invention relates to a hydraulic circuit for a backhoe including a load sensing system for controlling pump flow amounts according to a sensed load.
2. Description of the Related Art
The above-described type of hydraulic circuit for a backhoe having a load sensing system is known from the Japanese Patent Application “Kokai” No. 2000-206256. According to the backhoe hydraulic circuit disclosed by this reference, when the backhoe is to travel without being accompanied by any implement operation, pressure oils respectively from a first pump and a second pump are supplied independently to a left traveling valve section and a right traveling valve section for driving left and right traveling units. When only a front implement for excavating operation is to be operated with keeping the backhoe still (i.e. under non-traveling condition), the pressure oils from the first and second pumps are combined to be supplied together to a front implement valve section and also flow amounts of these pressure oils from the first and second pumps are controlled according to a hydraulic load during the front implement operation detected by the load sensing system. Further, when the front implement is to be operated while the backhoe keeps traveling, the pressure oils from the first and second pumps are supplied independently to the left and right traveling valve sections and also a pressure oil from a third pump provided for swiveling and dozer operations is also supplied to the front implement valve section.
With the above-described conventional hydraulic circuit, however, since the combined oil from the first and second pumps is supplied to the front implement, the maximum flow amount of each of the first and second pumps is about a half of the maximum flow amount required for a normal front implement operation. For instance, in the case of a backhoe of 5 tons class, the maximum flow amount required for front implement operation is about 130 liters/min. Hence, the maximum flow amount from each of the first and second pumps is about 65 liters/min, so that the combined maximum flow amount from the two pumps exceeds a flow amount (generally, 45 to 50 liters/min) required for traveling of the vehicle of this tonnage class.
Accordingly, in the first-mentioned case when the vehicle is to travel without any implement operation, with supply of the pressure oils to the left and right traveling unit valve sections, a flow-amount control scheme based on power control will cause the first and second pumps to output flow amounts which, when combined, exceeds the required amount for traveling, thus tending to invite overheating and/or excessive rise in temperature of the work oil. Further, in the third-mentioned case when the traveling of the backhoe and the operation of the front implement are to take place at the same time, a selector valve is needed for guiding the pressure oil from the third pump to be supplied together with the other pressure oils to the front implement valve section. The provision of such valve invites cost increase.
In addition, in the case of the above-described backhoe hydraulic circuit, if a swiveling operation of a swivel table and a lift-up operation of a boom for the front implement are effected at one time, a significant inertia of the swivel table at the time of its activation will cause a rise in the start-up pressure, so that the flow amounts of the first and second pumps are reduced by the pump power control scheme, thus resulting in disadvantageous reduction in the rising speed of the boom.